This Christian symbol looks like our capital letters "O W N."
The letters on a nimbus are often an Omicron, Omega and Nu as seen to the left. These letters are (very) roughly translated to mean "the one who is." This is a reference to the Greek translation of Exodus 3.14, in which Moses asks for God's name. God's response is difficult to translate, and is often rendered something like "I am what I am" or "I am the one who is." That phrase is signified here by the Omicron, Omega and Nu.
When these letters appear as part of a nimbus behind Jesus, it is an affirmation of Christ's divinity and unity with the First Person of the Trinity.
This symbol is quite rare. It is worth looking closely at any nimbus to see if these letters appear. In my personal experience it is most prevalent in churches with Greek heritage.
Notice that the positions of the letters can sometimes shift, but the meaning is the same as long as they are the same three letters.
First, a background story is helpful.
One of the controversies leading up to the council of Nicea, one of the major early Christian councils, involved a theologian named Arius. To condense some of his arguments into a phrase, Arius held the position that "there was when He was not" - meaning that there was a time before Jesus existed.
Athanasius represented the opposite position, claiming that Jesus Christ is eternal. His side is sometimes condensed to the phrase "there was not when He was not."
Ultimately, the Church sided with Athanasius, declaring that Jesus Christ always is, always was, and always will be.
After considering this story, read the following passages:
John 1 (remember that "the Word" refers to Jesus).
Given the passages from John and Revelation, why would artists want to connect Jesus to Exodus 3.14?
How does this influence your understanding of Jesus?
The Nimbus with the Omicron Omega Nu provides a way to discuss Christology and to connect Old Testament passages to new Testament passages. It also provides an opportunity to talk about the Trinity
Due to its emphasis on Jesus and the his existence throughout time, this symbol could work well close to Christmas, particularly when paired with John 1. It could also work at any point in ordinary time.